Workshop on Strain and Gradient Plasticity: Applications and Theory

John W. HutchinsonSchool of Engineering and Applied Sciences, Harvard University

(Lectures will be held over the Access Grid from UCSB)

Plastic deformation at the micron scale is assuming increasing importance in areas of technical application, including MEMS, thin films and coatings, metal-matrix composites with micron sized reinforcing particles, and in attempts to understand fundamentals of material fracture. Recent experiments have revealed aspects of micron scale deformation that are not present in bulk, or conventional, plasticity. Of major importance is a strong size dependence whereby the effective yield strength increases with decreasing specimen size when non-uniform straining occurs. The phenomenon is revealed by indentation hardness tests, by bending and torsion tests, by straining of thin films bonded to a substrate, and by the well-known dependence of polycrystalline yield stress on grain size. In effect, smaller is stronger when deformation involves gradients in the plastic strain.

The lectures will begin with an overview of a broad selection of recent experimental data that motivates the need for plasticity theories applicable at the micron scale. The underlying physical basis of the theory will also be addressed. The review of experimental data will be followed by the introduction of a phenomenological strain gradient plasticity theory which reduces to the most commonly used conventional plasticity theory (so-called J2 flow theory) when the scale of the deformations is not small. Illustrative examples based on the theory will be presented. Some discussion of numerical approaches will be given because numerical methods are even more essential for generating solutions based on strain gradient theories than for conventional plasticity. The lectures will conclude with a discussion of open issues and prospects for the future.